The present invention relates to joysticks and, more particularly, to a two-dimensional position sensor using a movable shaft, incorporating a conductive material, surrounded by at least three, curved triangular-shaped sense coils. The physical position of the shaft is determined by triangulation, by detecting the change in the self-inductance of the sense coils as the shaft moves.
Joysticks are used in many military, industrial, and commercial applications to control movement of an aircraft, vehicle, object on a video screen, etc. Most joysticks convert the angular movement of a control shaft into movement along an X and Y axis, using a mechanical linkage to translate the motion. The displacement of the shaft in each direction is detected by means of mechanical switches, variable resistors, or optical sensors. Usually the greater the resolution required in detecting the shaft position, the greater the cost of the precision sense elements, such as optical sensors, required to detect tiny changes in the shaft position.
A number of joystick systems using inductive sense elements have been developed. U.S. Pat. No. 4,685,678 describes a joystick system where position is determined through use of a pair of inductors that operate with a movable slug. The movement of the slugs, by the joystick handle, causes a change in inductance. The inductors produce signals proportional to the position of the slugs in two dimensions.
U.S. Pat. No. 4,855,704 describes a system that utilizes two induction coils, and a spherical induction body secured to the joystick. As the joystick is moved, the location of the induction body relative to the sensors changes the inductance of the sensors.
Finally, U.S. Pat. No. 5,598,090 describes a joystick system that uses biasing springs as inductors. The biasing springs position the joystick in a neutral position. Movement of the joystick compresses or extends the springs, changing their inductance. All of these approaches incorporate multiple, movable components, or mechanical elements to translate the joystick position into two dimensions.
U.S. Pat. No. 5,949,325 describes a joystick system where the joystick is secured to a conductive rubber transducer. As the joystick is moved about, the curved rubber transducer is deflected and contacts conductors on a printed circuit board. This approach eliminates many of the mechanical moving parts required to translate angular motion into two axes, however, it uses direct contact between the joystick and sensing elements.
In view of the above, there is a need in the art for a joystick with no moving parts other than the joystick shaft. Further, it would be desirable to eliminate the need to mechanically translate the motion of the shaft into an X and Y direction. It would be further desirable to detect the position of the joystick shaft without physical contact to the shaft which, in combination with the elimination of moving parts, provides improved reliability and durability. It is also desirable to detect the joystick shaft position with significant precision to provide increased resolution using low cost sensors.
The needs described above are in large measure met by an inductive joystick of the present invention. The inductive joystick has no moving parts other than the joystick shaft, and utilizes low-cost printed sense inductors. The shaft position is detected by use of a triangulation algorithm that can detect at least 72 discrete positions within the sphere of movement of the shaft. There is no physical contact with the joystick shaft.
Specifically, the inductive joystick of the present invention includes a shaft that is provided at a first end with a layer of a cone-shaped conductive material. A tubular housing surrounds the first end and contains at least three curved sense inductors that surround the movable shaft. The sense inductors are preferably triangular in shape. The distance of the movable shaft from the sense inductors varies as the shaft is moved in a two dimensional plane. The variation in distance between the conductive material and the sense inductors causes variation in the inductance of each of the triangular shaped inductors and this variation in inductance is used to determine the physical position of the movable shaft through use of triangulation calculations.